<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>

</body>
<script>
   // 将div#demo.demo转化成<div id="demo" class="demo"></div>
   const str = 'div#demo.demo';
   const reg = /(\w+)\#(\w+)\.(\w+)/;
   var s1 = str.replace(reg, function(match,$1,$2,$3){
      console.log($1, $2, $3);
      return "<"+$1+" id=\"" + $2+ "\" class=\""+ $3+ "\"><\/" + $1 +">"
    });
    console.log(s1);
   
</script>
</html>